The combustion of fossil fuels such as coal, oil, industrial or natural gas produces environmentally hazardous substances including nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2). Nitrogen oxide and nitrogen dioxide are collectively called NO.sub.x. In the normal combustion process of fossil fuel, the major portion of NO.sub.x is NO.
As is well known, the production of NO.sub.x can occur when fossil fuel is combusted in a variety of apparatus. Accordingly, the current invention may have application in process and refinery heaters, gas turbine systems, and boilers including steam plants. The fuel may include coal, oil, gas, waste product such as municipal solid waste, and a variety of other carbonaceous materials. The invention applies to apparatus having particle-laden flue gas and having so-called "clean" flue gas.
A number of NO.sub.x reducing agents are known. Ammonia is commonly used. A principal process for the removal of NO.sub.x from the flue gas stream is the injection of a reducing agent such as ammonia, urea, or any of a number of other known reducing agents. For example, a very common method is the selective catalytic reduction (SCR) of NO.sub.x involving the injection of ammonia (NH.sub.3) into a flue gas and subsequent chemical reaction in the presence of catalyst; namely, ##STR1##
One traditional method of injecting ammonia into a flue gas stream uses an external ammonia vaporization system in which liquid ammonia, either in anhydrous or aqueous state, is first vaporized in a heater or vaporizer, mixed with air, and then routed to a distribution grid network for subsequent injection into the flue gas stream at a location upstream of an SCR reactor. A more detailed description of a known method and system for injecting anhydrous ammonia may be found in S. M. Cho, A. H. Seltzer, and Z. Tetsui, "Design and Operating Experience of Selective Catalytic Reduction Systems for NO.sub.x Control in Gas Turbine Systems," International Gas Turbine and Aeroengine Congress and Exposition at Orlando, Florida, Jun. 3-6, 1991 (ASME paper number 91-GT-26), hereafter called Cho et al. Because anhydrous ammonia is toxic and hazardous, the current "general" practice uses aqueous ammonia (NH.sub.3.H.sub.2 O), which is a mixture of ammonia and water. Since ammonia is diluted with "benign" water, aqueous ammonia is less hazardous than anhydrous ammonia. A typical industrial grade aqueous ammonia contains approximately 30%. ammonia and 70% water. The ammonia-water mixture of the above percentages is safely transported on U.S. highways. It has a negligible vapor pressure at ordinary temperature.
Also known is a process that does not employ a catalyst, the so-called selective non-catalytic reduction (SNCR) process. Ammonia, urea, or other reducing agent is injected into the upper combustion area of a furnace or other combustor. Other injection sites are known, including the cyclone separator of a circulating fluidized bed steam generator.
In those systems and methods that use aqueous ammonia, there are several methods that are currently used to vaporize the ammonia. These include: (1) the use of ar electric heater to heat ambient air and mix it with aqueous ammonia in a vessel, thus vaporizing the aqueous ammonia (described in Cho et al.), (2) the use of a kettle-type heat exchanger tank in which a tank filled with aqueous ammonia contains coils that are supplied with steam to vaporize the aqueous ammonia, (3) the use of an ammonia stripping tower in which aqueous ammonia is sprayed into the top of a fluid-fluid type contact tower and steam is introduced into the bottom, and (4) the use of a flue gas slip stream that is drawn by a blower into a vaporizer vessel where the flue gas mixes with and vaporizes aqueous ammonia.